Liquid scintillation counting is an analytical technique by which the energy of radioactive emissions from isotopes used for labeling a select material, or intrinsically present in a sample, is converted into light so that the isotopes can be detected and measured. By detecting and measuring light caused by the radioactive emissions, the emissions can be analyzed to determine quantitative information about the isotopes in the sample.
Liquid scintillation depends on the critical interactions between the radioactive sample and the surrounding liquid solution medium. This solution is often referred to as a "cocktail." The emission of radioactive energy from the radioactive sample is accompanied by an excitation, or "ionization" of the molecules in the cocktail. Certain molecules in the cocktail emit (typically) visible light, i.e., they "fluoresce", when they interact with ionizing radiation. The liquid scintillation process by which radioactive emissions are investigated is, then, an analysis or investigation of an energy transfer process in which at least two organic compounds in a scintillation solution participate. It is the function of the liquid scintillation solution to convert the energy of the emitted radioactive particles into light which can be detected and measured.
A "liquid scintillation counter" is an instrument used to detect the emitted light flashes, or "light scintillation", of the fluorescence. The rate of production of light scintillations is proportional to the rate of radioactive decay. Additionally, the intensity of the light scintillations (measured as "photons") is proportional to the energy of the decaying radioactive particles. Exemplary of such instruments is the LS 6000.TM. Series Scintillation Systems, available from Beckman Instruments, Inc. (Fullerton, Calif. 92634, U.S.A.).
The chemical nature of the liquid scintillation solution facilitates energy transfer from the radioactive emissions to constituents in the liquid scintillation solution causing scintillation. The liquid scintillation solution comprises a chemical known as a "fluor," also referred to as a "primary scintillator," which is dissolved in another organic chemical known as a "solvent." The fluor is the chemical constituent of the liquid scintillation solution which emits light by fluorescence when sufficient energy is received via the solvent from a radioactive particle emitted by the isotope. Common beta-emitting isotopes include tritium (.sup.3 H), carbon-14 (.sup.14 C) and chlorine-36 (.sup.36 Cl). A common alpha emitting isotope includes Americium-241 (.sup.241 Am). The solvent (alone) does not scintillate; rather, it functions as a "link" in the energy transfer "chain" by passing on its acquired energy to the fluor which in turn permits light scintillation to occur.
A beta particle traveling through the liquid scintillation solution following emission from a radionuclide causes excitation of the solvent molecules surrounding the labeled sample molecule in the scintillation medium. As an emitted beta particle travels through the scintillation medium, molecules of the solvent become converted to excited molecules as the radioactive particle loses its energy to these molecules. The amount of energy which the emitted beta particle possesses determines how far the particle will travel before it comes to rest. Because the beta particle loses energy by interaction or contact with the solvent molecules in its path (when the solvent molecules become excited to higher energy states), the number of solvent molecules that the emitted beta particle will excite is directly proportional to the distance the particle travels through the liquid scintillation solution. This in turn is proportional to the energy the particle possesses.
A typical molecule of a solvent which can be excited by coming in contact with a radioactive particle is aromatic in chemical character. An "aromatic" chemical compound has at least one benzene ring. The aromatic character of the molecule permits electrons to become excited to a higher energy state so that the solvent molecule can retain the excitation energy for a period of time. Detailed information regarding the characteristics of solvents applicable in liquid scintillation cocktails are set forth in, inter alia. U.S. Pat. No. 4,867,905, which is incorporated herein by reference.
Once the solvent molecule is excited by absorbing energy from the emitted radioactive particle, it can lose this energy in one of two modes, emission of heat or light. Before the solvent molecule can decay, thus relieving its absorbed energy, it is desirable that a fluor molecule in the scintillation solution interact with the solvent molecule. The presence of a fluor allows for the eventual transfer of excitation energy of the solvent molecule to the fluor molecule. The fluor molecule then releases its excitation energy as light. Detection and measurement of energy received by the fluor is accomplished by detecting and measuring the light generated.
A widely known fluor utilized in liquid scintillation systems is 2,5-diphenyloxazole ("PPO"). Other applicable fluors include 2-(4'-biphenylyl)-6-phenylbenzoxazole ("PBBO"); 2 (4 tert.-butylphenyl) 5-(4"-biphenylyl)-1,3,4-oxadiazole ("Butyl-PBD");2-(1-naphthyl)-5-phenyloxazole (".alpha.NPO"); 2-(4-biphenylyl)-5-phenyl-1,3,4-oxadiazole ("PBD"); 9,10-diphenylanthracene ("DPA"); 2,5-bis (5'-tert.-butyl-2-benzoxazole) thiophene ("BBOT"); P-bis (o-methylstyryl) benzene ("BisMSB"); and 2,2'-p-phenylenebis [5-phenyloxazole] ("POPOP").
A fluor is generally present in the liquid scintillation solution in very low concentrations, on the order of five to ten grams per liter. Thus, solvent molecules which have been excited by absorption of energy from a beta particle often transfer their energy to another solvent molecule which in turn becomes excited as the transferring solvent molecule is de-energized. This energy transfer chain, or "cascade", can continue until an excited solvent molecule transfers this energy to a fluor molecule, which then will decay by scintillating.
Light released by a fluor is generally in a narrow frequency or wavelength band. The number of photons of light energy released is directly proportional to the number of solvent molecules from which the fluor molecules receive energy over a certain period of time. The energy transferred to an excited solvent molecule ultimately energizes a fluor molecule, which then releases light by scintillation. This permits the energy of the radioactive particle to be measured as intensity of light because high energy particles will have long path lengths and excite many solvent molecules, which then excite fluor molecules. Because the scintillation process takes place in a matter of nanoseconds, the number of fluor molecules fluorescing within such a small segment of time will be detected as greater or less intensity of light by, e.g., a photomultiplier tube(s) in a liquid scintillation instrument. Thus, high energy radioactive particles emitted from the isotope will have long pathlengths and excite many solvent molecules which in turn cause fluor molecules to fluoresce many times releasing bright light; low energy particles, on the other hand, will have shorter pathlengths and excite fewer solvent molecules which in turn cause fewer fluor molecules to fluoresce and release dimmer light.
In some instances a secondary fluor, also referred to as a "secondary scintillator", will be used to shift the wavelength region of light scintillation caused in the liquid scintillation solution to one which is more desirable. Secondary fluors are also organic compounds which are added to the liquid scintillation solution in small quantities relative to the quantity of the primary fluor. The light scintillation of a secondary fluor is caused by energy received from the primary fluor. The result of having a secondary light scintillation is to shift emitted light to longer wavelengths. An exemplary secondary scintillator is 1,4-di-(2-methylstyryl)-benzene ("Bis-MSB").
Recent increased attention and focus on, inter alia, the environmental impact of radioactive materials from naturally occurring radioactive sources (e.g., radon gas) and from human-made radioactive materials and by-products (e.g. from nuclear-reactor based energy systems) has lead to a greater need to obtain accurate, quantitative information regarding both alpha- and beta-emitting particles. Briefly, an "alpha" radioactive decay refers to the emission of a helium ion ("He.sup.+2 ") from an unstable nucleus to produce a different nuclide with four less atomic mass units and two less protons in the nucleus. (A "nuclide" refers to a particular nuclear species of an element characterized by the number of protons and neutrons). For example, schematically an alpha radioactive decay can be set forth as follows vis-a-vis Americium ("Am") and Neptunium ("Np"): ##STR1## Characteristically, alpha particles have discrete, well-defined energy spectrum. Beta particles, on the other hand, have very broad energy spectrum, due to energy sharing between the released beta particle and an anti-neutrino .upsilon.. Normally, the energy spectrum begins at zero kilo-electron Volts ("keV") where all of the energy is given to the anti-neutrino, and ends at some energy maximum keV based upon the particular radionuclide. With beta-decay, the mass number of the nuclide remains the same, while the number of protons increase. Schematically, a beta radioactive decay can be set forth as follows vis-a-vis Chlorine ("Cl"), and Argon ("Ar"): ##STR2## For convenience, a liquid scintillation counter spectrum for .sup.241 Am and .sup.36 Cl are set forth in FIG. 1 in terms of counts per minutes versus keV.
Alpha particles do not (generally) penetrate the epidermal layer of tissue (e.g., the outer layer of animal skin tissue). Alpha particles ionize other molecules more efficiently than beta-particles, and thus are more insidious than beta emitting particles. Once alpha particles enter an animal by, e.g., airborne intake via the lungs, they can directly damage the lung tissue. Unlike alpha particles, beta emitting particles can penetrate tissue; as such, they can damage tissue directly.
Accordingly, it is essential to be able to accurately define and quantify the alpha- and beta-emitting particles of a particular radioactive material. Radon gas, for example, which has received widespread attention over the past decade, will spontaneously decompose in about four hours to produce three alpha-particles and two beta-particles. In certain geographical regions of the United States of America, for example, it is essential that dwelling facilities be regularly checked for radon gas, given the potential for this naturally occurring material to potentially cause radiation-related damage.
Previous methodologies for discriminating between alpha- and beta-emitters have been proposed. For example, the energy range for most beta emitters is 0-2 million eV ("MeV"), while for alpha-emitters, the range is from 3-8 MeV. These differences can (somewhat) be used for discrimination purposes, although a range of overlap between the high end of the beta-emitters and the low end of the alpha-emitters is possible. Alpha- and beta-emitters mixed in the same sample can be discriminated from each other by the time distribution of the light emission each generates from the liquid scintillation cocktail, typically referred to as "pulse shaped discrimination" or "PSD".
PSD is predicated upon both a fast and a slow decay component of a light pulse generated over a period of time; the fast component has an exponential decay time on the order of about 1-10 nanoseconds ("ns"), while for the slow component, 200-350ns, each being dependent upon the specifics of the particular liquid scintillation solution. The fast component is the result of normal fluorescence while the slow component results from delayed fluorescence. The wavelength of both components is the same. There is a difference in the relative distribution of the fast and slow components for alpha and beta particles; with alpha emitters, the slow component predominates, while with beta emitters, the fast component predominates. This difference can be exploited for alpha-beta discrimination via PSD and the so-called "R-value technique." The R-value is derived by integrating the area under portions of a light-intensity versus time plot of light pulses measured at two different times. I.e., t.sub.0 .fwdarw.t.sub.1 =I.sub.1 and t.sub.0 .fwdarw.t.sub.2 =I.sub.2. A ratio value, R, is then derived according to the formula R=I.sub.2 /I.sub.1, where R is the ratio of the two integrated areas. By determining several R values for both the alpha- and beta-emitters in the sample, an "R-value spectrum" can be developed for the emitters. FIG. 2 provides an R value spectrum for .sup.241 Am and .sup.36 Cl using a previous liquid scintillation cocktail comprising PBBO, naphthalene and toluene used for alpha-beta discrimination.
Using such cocktails, the R value spectrum, when used for alpha-beta discrimination, evidences a "contamination" region where an overlap occurs. Such a region is set forth in FIG. 2 at approximately an R value minimum of 1.4; this is a region where discrimination between alpha- and beta-emitting particles is not possible. For this particular analysis, this region represents a 5% beta-emission contamination. Typically, a 4 to 8% contamination range is considered acceptable using such previous cocktails. Clearly, however, this is a mere accommodation to the shortcomings of such cocktails in that ideally, the percent of contamination should approach zero.
Additionally, the utility of such cocktails is also evaluated based upon the "peak-to-valley ratio", or PV ratio, obtained from the R value spectrum. The PV ratio is defined by the unit value of the highest peak from the R value spectrum (i.e., typically the beta R value peak) divided by the lowest portion between the two peaks (i.e., in FIG. 2, the contamination region). For FIG. 2, the PV ratio is 4.4.
Preferably, an alpha-beta discrimination cocktail should completely discriminate between the two emitters; this, of course, would mean that there would be substantially little, if any, contamination region such that the peak-valley ratio would be quite high, i.e., much greater than the 4.4 value obtained from FIG. 2. Such a cocktail would allow for an accurate indication of the precise differentiation between the alpha-emitting components and the beta-emitting components derived from a single sample.